Method for testing a tire by interferometry

ABSTRACT

A method for testing a tire by interferometry in a pressure chamber of a tire testing device includes capturing phase images at different pressures in the pressure chamber, generating partial phase difference images between successive phase images, and summing the partial phase difference images to form an overall phase difference image. The pressure in the pressure chamber is changed in a first direction during a first measurement phase and the pressure is changed in the opposite direction during a second measurement phase, wherein at least one partial phase difference image from the first measurement phase and at least one partial phase difference image from the second measurement phase are included in the summation.

TECHNICAL FIELD

The present disclosure relates to a method for testing a tire byinterferometry in a pressure chamber of a tire testing device, and acorresponding tire testing device. The present disclosure furtherrelates to a testing method based on shearography.

BACKGROUND

Using methods for testing a tire by interferometry in a pressurechamber, it is possible to identify defects, such as air inclusions,lying in the interior of the tire. To this end, the pressure in thepressure chamber of the tire testing device is changed and thedeformation of the tire generated by the pressure change is measured byinterferometry. Phase images of the surface of the tire to be tested arecaptured at at least two different pressure states and a phasedifference image is formed from the phase images, said phase differenceimage showing the deformation of the tire.

The interferometric test can be a holographic test or a shearographictest. However, use is typically made of shearography as it has thedecisive advantage over holography that it can also be used inindustrial conditions and in industrial surroundings.

However, two substantial interfering factors still exist when testingtires by interferometry, even when using shearography: This is, interalia, a whole-body deformation of the tire during the test, caused by apreceding deformation, a local deformation of the tire during the test,caused by a preceding spatial restricted deformation, and vibrations ofthe tire during the test. The whole-body deformation of the tire isusually based on deformations of the tire which were generated, forexample, by introducing the tire into the tire testing device and therelaxation of the tire back into its initial state, occurring during thetest. Local deformations of the tire are also usually based on precedinglocal deformations, for example in the form of pressure points, whichrelax during the test. Vibrations of the tire can be excited by verydifferent effects, for example by air flows when changing the pressureor external vibration stimulators.

FIG. 1 shows a first measurement log known from the prior art. Here,negative pressure is generated initially during a first phase and thenegative pressure then is released again during a second phase by quickaeration of the pressure chamber. A first phase image B₀ is captured atmaximum negative pressure and a second phase image B₁ is captured uponreturn to the initial pressure after releasing the negative pressure.Then, a phase difference image D=B₁−B₀ is generated from the two phaseimages B₀ and B₁. This procedure is relatively insensitive in relationto vibrations of the tire. However, whole-body deformations have a veryproblematic effect here. In particular, these may be substantiallylarger than the actual deformations from defects of the tire to beexamined and/or these may lead to decoherence between the phase images.

Therefore, the document DE 10101057 A1 describes a further method fortesting tires by shearography, in which a sequence of phase images ofthe object is recorded during the deformation of the tire. Thereupon, apartial phase difference image is generated from respectively twosuccessive phase images and the individual partial phase differenceimages are summed to form an overall phase difference image. Anexemplary embodiment of such a method is shown in FIG. 2. Here, asequence of phase images B₀ to B₆ is captured while the negativepressure in the pressure chamber of the tire testing device isincreased. In each case, a partial phase difference imageD_(i)=B_(i−1)−B_(i) is determined between adjacent phase images B_(i)and B_(i−1), and then all partial phase difference images D_(i) aresummed to form an overall phase difference image.

As a result of summing a plurality of phase difference images, themeasurement method is relatively insensitive to whole-body deformations.In particular, the coherence between adjacent phase images ismaintained. Furthermore, the whole-body deformation can be eliminatedfrom the overall phase difference image. However, the interferencescaused by vibrations are amplified during this procedure. As a result,the tire testing devices become more susceptible to errors, the overallphase difference images become noisy and the test times are increased.

SUMMARY

It is an object of the present disclosure to provide an improved methodfor testing a tire by interferometry, and a corresponding tire testingdevice.

The present disclosure includes a method for testing a tire byinterferometry in a pressure chamber of a tire testing device, includingthe steps of:

-   -   capturing phase images at different pressures in the pressure        chamber;    -   generating partial phase difference images between successive        phase images, and    -   summing the partial phase difference images to form an overall        phase difference image.

Typically, the phase images are generated by a shearographicmeasurement. According to an aspect of the disclosure, the pressure inthe pressure chamber is changed in a first direction during a firstmeasurement phase and there is a change in the pressure in the oppositedirection during a second measurement phase, wherein at least onepartial phase difference image from the first measurement phase and atleast one partial phase difference image from the second measurementphase are included in the summation. Therefore, if the absolute pressurein the pressure chamber is reduced during the first measurement phase,it is increased during the second measurement phase, and vice versa.

Since, when proceeding, e.g., from atmospheric pressure, two phases withan opposite pressure profile are necessary in any case to be able toremove the tire at atmospheric pressure again, the procedure accordingto an aspect of the disclosure is advantageous in that both phases canalso be used for measurement purposes. By contrast, one phase remainsunused in prior art. Furthermore, the inventors of the presentdisclosure identified that not only is it possible to use both phases asmeasurement phases but that, moreover, partial phase difference imagesfrom both the first measurement phase and the second measurement phasecan be included in the overall phase difference image. Since, accordingto an aspect of the disclosure, this means that two measurement phaseswith opposite changes in pressure are included in the overall phasedifference image, there is a corresponding increase in the change inpressure effectively considered for creating the overall phasedifference image. Hence, there is an increase in the quality of the tiretest in the case of the same change in pressure since faults emerge moreclearly. Alternatively, the change in pressure can be reduced inrelation to the prior art with the quality remaining unchanged. This hassignificant advantages in terms of costs since the tire testing deviceonly still need be designed for a smaller pressure. Moreover, themeasurement time is shortened and the problems with vibrations arereduced.

In an exemplary embodiment of the present disclosure, provision is madefor the partial phase difference image or the partial phase differenceimages from the second measurement phase to be included in the summationwith an opposite sign. As a result, it is possible to take account ofthe opposite change in pressure during the second measurement phase inrelation to the first measurement phase and hence to take account of theinverse deformation of the tire.

The procedure according to an aspect of the disclosure has the furtheradvantage that interfering influences resulting from a whole-bodydeformation, from local deformations such as pressure points and fromvibrations are reduced. Usually, these interfering influences haveapproximately the same behavior over both measurement phases—unlike thedeformation generated by the opposite change in pressure on account offaults in the tire. However, since the partial phase difference imagesfrom the second measurement phase are included with the opposite sign inthe summation, the interfering influences from the second measurementphase therefore at least partly cancel the interfering influences fromthe first measurement phase.

In an exemplary embodiment of the present disclosure, provision is madefor at least two partial phase difference images to be determined, andtypically included in the summation, during the first measurement phaseand/or the second measurement phase. According to an aspect of thedisclosure, there therefore continues to be a summation over a pluralityof partial phase difference images even within the individualmeasurement phase. As a result, it is nevertheless possible to obtain ahigh quality of the test despite relatively large overall deformations.

Typically, the time duration of the second measurement phase is at least50% of the time duration of the first measurement phase. This ensures anat least partial cancellation of the interfering influences from thefirst measurement phase and second measurement phase. The measurementtime duration of a measurement phase is typically determined as the timeinterval between the generation of the first phase image included in apartial phase difference image of this measurement phase and the lastphase image included in a partial phase difference image of thismeasurement phase. Typically, the time duration of the secondmeasurement phase is at least 90% of the time duration of the firstmeasurement phase. By way of example, the second measurement phase mayhave exactly the same time duration as the first measurement phase.

However, since the deformations of the tire on account of interferinginfluences tend to reduce over time, the second measurement phase mayalso have a greater time duration than the first measurement phase. Byway of example, the time duration of the second measurement phase may beat least 110% of the time duration of the first measurement phase.

Typically, the time interval between the generation of successive phaseimages over the first measurement phase and second measurement phasevaries by at most 20%, typically by at most 10% of the shortest timeinterval. Particularly typically, the time interval between thegeneration of successive phase images over the first measurement phaseand second measurement phase is the same for all phase images. As aresult, the influence of vibrations on the overall phase differenceimage can be reduced since interferences from the first measurementphase and the second measurement phase at least partly cancel oneanother.

Alternatively, the time interval between the generating of successivephase images over the first measurement phase and second measurementphase may vary, in particular may vary in the same way and/or in astatistical manner. This too can reduce the influence of vibrations onthe overall phase difference image.

By way of example, a time interval of between 0.01 s and 1 s, moretypically between 0.05 and 0.5 s, can be selected between the generatingof successive phase images. By way of example, each frame or each n^(th)frame from the video recording rate of the sensor can be used as ameasurement point, where n typically lies between 2 and 10.

In an exemplary embodiment of the present disclosure, provision is madefor the same pressure to prevail in the pressure chamber during thefirst generating of a phase image during the first phase and during thelast generating of a phase image during the second phase. In particular,the initial state of the first phase may be atmospheric pressure andthere may be a return to atmospheric pressure during the second phasesuch that loading the tire testing device with the tire prior to thefirst phase and removing the tire from the tire testing device after thesecond phase may be carried out at atmospheric pressure in each case.

Here, work can typically be carried out at negative pressure such that,during the first measurement phase, negative pressure is generatedproceeding from atmospheric pressure and said negative pressure isincreased over the measurement phase. Then, the negative pressure isreduced again during the second measurement phase, typically back toatmospheric pressure. Accordingly, the pressure chamber can be anegative pressure chamber. However, exemplary embodiments that operateusing positive pressure are also conceivable.

Here, according to an aspect of the disclosure, it is not necessary forgenerating the phase images to extend over the entire change inpressure. By way of example, the first phase image of the firstmeasurement phase can be generated only after a certain change inpressure has been undertaken. Conversely, it is also possible for thelast phase image of the second measurement phase to be generated and fora change in pressure to still be undertaken thereafter.

In particular, the first measurement phase therefore may only set inafter a change in pressure in relation to the initial pressure in thepressure chamber was undertaken and/or wherein the second measurementphase ends before the pressure in the pressure chamber has returned tothe initial pressure.

Such an offset of the measurement phase or measurement phases inrelation to the initial pressure may reduce interfering influences.

However, in an exemplary embodiment, the first phase image during thefirst phase and the last phase image during the second phase aregenerated at atmospheric pressure.

The more similar the pressure profiles are during the first phase andduring the second phase, the better interfering influences, which arebased on vibrations, can be compensated by the summation over bothphases.

Therefore, an exemplary embodiment of the present disclosure providesfor the change in pressure during the second measurement phase to beeffectuated with an average speed which deviates by at most 20% andtypically by at most 10% from the average speed of the change inpressure during the first measurement phase.

Furthermore, an exemplary embodiment of the present disclosure providesfor the first phase image during the first phase and the last phaseimage during the second phase to be recorded at the same pressure.

However, according to an exemplary embodiment of the present disclosure,the change in the pressure during the second measurement phase iseffectuated more quickly than the change in the pressure during thefirst measurement phase. By way of example, there can be quick ventingor quick aerating of the pressure chamber during the second measurementphase.

This takes account of the fact that many commercially available deviceshave such a structural design that the negative pressure during thefirst phase increases more slowly than it falls during the second phase.The present disclosure can also be applied in the case of these devices.If the pressure profile is maintained in the process, a software updateis all that is required.

If the pressure during one of the two measurement phases changes morequickly than in the other measurement phase, this measurement phasetypically includes a portion during which the pressure does not change.As a result, the otherwise present difference in the time duration ofthe measurement phases can be reduced.

However, provision is also made for the change in the pressure duringthe second measurement phase to be effectuated in symmetrical fashionwith respect to the change in the pressure in the first measurementphase. As a result, it is possible to compensate to the best possibleextent interfering influences that are based on vibrations.

In a further exemplary embodiment of the present disclosure, provisionis made for a plurality of successive first and second measurementphases. Typically, a summation is effectuated here over the partialphase difference phase images from all measurement phases for thepurposes of producing the overall phase difference image. As a result,it is possible to reduce even further the change in pressure permeasurement phase and hence the compressive strength of the tire testingdevice required overall.

Typically, the absolute pressure changes during a plurality ofsuccessive measurement phases have the same magnitude.

Typically, the successive first and second measurement phases form aperiodic pressure profile. This once again reduces interferences in thetest result which are based on vibrations.

In an exemplary embodiment of the present disclosure, provision is madefor at least one phase image to include both a partial phase differenceimage from a first measurement phase and a partial phase differenceimage from a second measurement phase. In particular, a phase image canbe generated at the transition between two measurement phases, saidphase image being included in the last partial phase difference image ofthe measurement phase lying before the transition and in the firstpartial phase difference image of the phase lying after the transition.By way of example, a phase image may be generated at the end of thefirst phase, during which, e.g., maximum negative pressure or positivepressure prevails, said phase image being included in the last partialphase difference image of the first measurement phase and in the firstpartial phase difference image of the second measurement phase. If useis made of a plurality of successive first and second measurementphases, it is also possible for a phase image to be generated at the endof the second measurement phase, for example at minimal negativepressure or positive pressure, said phase image being included in thelast partial phase difference image of the second measurement phase andin the first partial phase difference image of the first measurementphase.

In an exemplary embodiment of the present disclosure, provision is madefor negative pressure to be generated in the pressure chamber during thefirst measurement phase, said negative pressure typically being releasedagain during the second measurement phase. However, work mayalternatively also be undertaken with positive pressure.

In an exemplary embodiment of the present disclosure, a whole-bodydeformation is eliminated from the phase images and/or partial phasedifference images and/or the overall phase difference image. Inaddition, the whole-body deformation is determined from the overallphase difference image by filtering, said whole-body deformation thenbeing subtracted from the overall phase difference image.

In an exemplary embodiment of the present disclosure, the partial phasedifference images are phase filtered prior to the summation of thepartial phase difference images. This increases the quality of themeasurement results.

Typically, the phase images are captured a single measurement image. Asa result, the phase images may also be captured while there is a changein the pressure in the pressure chamber and while the tire isconsequently being deformed. By contrast, if a plurality of measurementimages are required to generate a phase image, the pressure in thepressure chamber must be kept constant during the recording of theplurality of measurement images so as not to deform the tire.

Typically, a spatial phase shift is used for generating a partial phasedifference image. Compared to a temporal phase shift, this isadvantageous in that the partial phase difference image can be generatedusing only one measurement image.

Typically, an imaging optical unit is used to generate a measurementimage, said imaging optical unit having a stop with at least oneaperture, in particular one slit, and typically two apertures, inparticular two slits, by which a spatial carrier frequency can begenerated. This facilitates generating of the phase images from only onemeasurement image in each case. In particular, use can be made of ameasurement arrangement as described in DE 198 56 400 A1, the content ofwhich, in its entirety, is incorporated in the subject matter of thepresent application.

In an exemplary embodiment of the present disclosure, provision is madefor the change in the pressure during the first phase and/or during thesecond phase to be effectuated continuously. The continuous change inpressure reduces induced vibrations. Typically, the course of the changein pressure during the first phase and/or during the second phase issubstantially linear. However, in an exemplary embodiment, the firstmeasurement phase or the second measurement phase may also include aportion during which there is no change in pressure.

In an exemplary embodiment of the present disclosure, provision is madefor at least one of the phase images to be recorded while the pressureis changed. This facilitates a quick measurement routine.

Typically, the change in pressure during the tire test according to anaspect of the disclosure is less than 50 mbar, typically less than 40mbar, further typically less than 30 mbar, further typically less than20 mbar, further typically less than 10 mbar. As already described inmore detail above, the procedure according to an aspect of thedisclosure facilitates a reduction in the change in pressure during theindividual measurement phases. As a result, the tire testing devicebecomes significantly cheaper since it only needs to be designed for asmaller pressure.

The present disclosure furthermore includes a tire testing device fortesting a tire by interferometry, having a pressure chamber, at leastone interferometric measuring head and a controller which is configuredin such a way that it carries out a method as described above.

Typically, the testing device furthermore has an apparatus for modifyingthe pressure in the pressure chamber. By way of examples, it may includeone or more pumps, valves, choke valves and/or flaps. In particular, usecan be made of a reversing valve.

Furthermore, the controller typically undertakes an evaluation of themeasurement results from the at least one measuring head and, inparticular, an evaluation of the measurement images measured by themeasuring head. The controller may be configured to capture phaseimages, to determine partial phase difference images and to generatingan overall phase difference image.

In particular, the controller can carry out the following method steps:

-   -   capturing phase images at different pressures in the pressure        chamber by actuating the measuring head and the apparatus for        changing the pressure in the pressure chamber;    -   generating partial phase difference images between successive        phase images; and    -   summing the partial phase difference images to form an overall        phase difference image.

Here, the controller changes the pressure in the pressure chamber in afirst direction during a first measurement phase and changes thepressure in the pressure chamber in the opposite direction during asecond measurement phase. Furthermore, at least one partial phasedifference image from the first measurement phase and at least onepartial phase difference image from the second measurement phase areincluded in the summation, undertaken by the controller, for generatingthe overall phase difference image.

Typically, the method for testing the tire is carried out in such a wayas already explained in more detail above.

Typically, the controller carries out the method automatically.

The controller typically includes a microprocessor and a memory, inwhich control software is stored, said control software running on themicroprocessor. The controller typically has a link via control linesand/or data lines to the measuring head and/or the apparatus forchanging the pressure in the pressure chamber. Furthermore, thecontroller typically has an output unit and/or an output interface.

The measuring head is typically a shearographic measuring head. Themeasuring head may have a laser, in particular one or more laser diodes,an imaging optical unit and an image sensor, in particular a CCD sensorand/or a CMOS sensor.

The measuring head typically operates on the basis of a spatial phaseshift. In particular, the measuring head may have an imaging opticalunit, said imaging optical unit having a stop with at least oneaperture, in particular one slit, and typically two apertures, inparticular two slits, by which a spatial carrier frequency can begenerated. This facilitates the generating of the phase images from onlyone measurement image in each case. In particular, use can be made of ameasurement arrangement as described in DE 198 56 400 A1.

The present disclosure furthermore includes control software for a tiretesting device, as described above. In particular the control software,if it is executed on a tire testing device, carries out a method asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will now be described with reference to the drawingswherein:

FIG. 1 shows a pressure profile and measuring points of a first testingmethod according to the prior art,

FIG. 2 shows the pressure profile and measuring points of a secondtesting method according to the prior art,

FIG. 3 shows the pressure profile and measuring points of a methodaccording to a first exemplary embodiment of the disclosure,

FIG. 4 shows the pressure profile and measuring points of a methodaccording to a second exemplary embodiment of the disclosure,

FIG. 5 shows the pressure profile and measuring points of a methodaccording to a third exemplary embodiment of the disclosure,

FIG. 6 shows the pressure profile and measuring points of a methodaccording to a fourth exemplary embodiment of the disclosure,

FIG. 7 shows the pressure profile and measuring points of a methodaccording to a fifth exemplary embodiment of the disclosure, and

FIG. 8 shows a schematic diagram of a tire testing device according toan exemplary embodiment of the disclosure.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

By way of example, the methods according to the exemplary embodiments ofthe disclosure may be carried out with the aid of a tire testing deviceas illustrated in a schematic diagram shown in FIG. 8. The tire testingdevice has a pressure chamber 2, into which the tire 1 is introduced forcarrying out the test. To this end, the pressure chamber 2 has at leastone opening, through which the tire can be introduced into the pressurechamber and/or be removed therefrom, said opening being sealable in anairtight manner. The tire testing device furthermore has an apparatus 3,4, by which it is possible to change the pressure in the pressurechamber 2. By way of example, provision can be made for a pump 3, bywhich negative pressure is generated in the pressure chamber 2.Furthermore, provision can be made of an apparatus 4 for releasing thenegative pressure, for example a valve, a choke valve, or a flap. Inparticular, use can be made of a reversing valve for the controlledchange of the pressure in the pressure chamber. However, otherapparatuses for changing the pressure in the pressure chamber 2 are alsoconceivable. Furthermore, work may also be carried out with positivepressure instead of with negative pressure.

The tire testing device furthermore has an interferometric measuringhead 5, by which at least a portion of the surface of the tire can betested. On account of the usually relatively small measuring field ofthe measuring heads, only a portion of the surface of the tire isusually tested during a testing process. Typically, a testing methodaccording to an aspect of the disclosure is carried out for each portionto be tested of the surface of the tire. Depending on the configurationof the tire testing device, it is possible to move the measuring headand/or the tire in order to test different portions of the surface ofthe tire in succession. By way of example, the tire 1 may lie on arotatable bearing 7 or the measuring head may be rotatable.Alternatively, a standing arrangement of the tire within the tiretesting device is also conceivable.

The tire testing device furthermore has a controller 6, by which theapparatus 3, 4 for changing the pressure in the pressure chamber 2 andthe measuring head 5 are actuated. The controller 6 furthermore servesto evaluate the data generated by the measuring head 5. The controlleris configured in such a way that it carries out a method according to anaspect of the disclosure by an appropriate actuation of the apparatusfor changing the pressure in the pressure chamber, by an actuation ofthe measuring head 5, and by an appropriate evaluation of the datagenerated. In particular, the controller carries out the methodautomatically in this case.

According to an exemplary embodiment of the disclosure, the pressure inthe chamber is changed after introducing the tire 1 into the pressurechamber 2 in order to capture a sequence of phase images at differentpressures in the pressure chamber. Now, partial phase difference imagesare determined from successive phase images, said partial phasedifference images accordingly corresponding to a partial deformation ofthe tire between the production of the two phase images. The partialphase difference images are then summed to form an overall phasedifference image.

Unlike methods according to the prior art, in which the pressure eitherrises or falls during the measurement phase, the method according to theexemplary embodiment of the disclosure includes two measurement phases,one with an increasing negative or positive pressure and one with afalling negative or positive pressure. At least one partial phasedifference image is determined for the first measurement phase and atleast one partial phase difference image is determined for the secondmeasurement phase, said partial phase difference images are then summedwith one another. In order to take account of the inverted pressureprofile during the second measurement phase, the partial phasedifference images generated therein are, however, included in thesummation with the inverse sign to the partial phase difference imagesgenerated during the first measurement phases.

FIG. 3 shows a first exemplary embodiment of such a method. Here, thenegative pressure is increased during a first measurement phase 10, andsaid negative pressure is reduced again during a second measurementphase 20. The phase images B₀ to B₅ are generated during the firstmeasurement phase, the phase image B₆ is generated in the overlap rangebetween the first measurement phase and the second measurement phase andthe phase images B₇ to B₁₀ are generated during the second measurementphase 20. The phase image B₆ consequently forms both the last phaseimage from the first measurement phase 10 and the first phase image fromthe second measurement phase 20.

A partial phase difference image D_(i)=B_(i+1)−B_(i) is generated ineach case from respectively successive phase images B_(i) and B_(i+1).These individual partial phase difference images D_(i) are then summed,with the phase difference images D₆ to D₉ of the second measurementphase 20 being included in the summation with the opposite sign to thepartial phase difference images Do to D₅ of the first measurement phase10. This results in an overall phase difference image which includesboth deformations of the tire during the first measurement phase anddeformations of the tire during the second measurement phase.

As a result of measurements being undertaken during both rising andfalling negative or positive pressure and as a result of thesemeasurements being included in the overall phase difference image, theeffective pressure change, and hence the effective deformation of thetire which is included in the overall phase difference image, doubles.The defects in the tire appear correspondingly clearer.

This can be used to reduce the changes in pressure used for testing thetire without impairing quality of the measurement result. Therefore, atire testing device according to an exemplary embodiment of thedisclosure can operate with a smaller positive or negative pressure andcan therefore also be generated in a correspondingly more cost-effectivemanner since, in particular, the pressure chamber only needs to bedesigned for a smaller negative or positive pressure. Moreover, thecomponents which are used for changing the pressure may have a simplerdesign.

Furthermore, the present disclosure also reduces the problems withvibrations since fewer vibrations are also excited on the tire onaccount of smaller changes in pressure. Furthermore, the method isaccelerated since it is now possible to use the entire pressure profilefor measurement purposes.

In the exemplary embodiment shown in FIG. 3, the phase images during thefirst measurement phase and during the second measurement phase arerecorded after the same time interval in each case. Furthermore, thephase images for the first measurement phase are recorded over timeperiod T₁ and the phase images for the second measurement phase arerecorded over time period T₂, with the time duration T₂ of the secondmeasurement phase being more than 50% and typically more than 80% of thetime duration T₁ of the first measurement phase. As a result, there isat least a partial elimination of problems caused by vibrations,pressure points and whole-body deformations.

In the exemplary embodiment shown in FIG. 3, which builds on thepressure profile that is known from the prior art illustrated in FIG. 2,the pressure change in the first phase 10 is slower than in the secondphase 20. This is the case in many devices according to the prior art,often caused by the mechanical structure of the apparatus, used therein,for changing the pressure in the pressure chamber. A method according toFIG. 3 therefore may be implemented by a software update without havingto modify the mechanical structure of the device, even in the case ofsuch apparatuses known from the prior art. As is clear from FIG. 3, thisleads to a change in pressure only being carried out over a firstportion of the second measurement phase 20. By contrast, there is nochange in pressure anymore over a second portion. Therefore, the furtherphase images B₉ and B₁₀ no longer contribute to identifying a geometricchange in the tire on account of faults of the tire. However, theycontribute to reducing the problems caused by vibrations, pressurepoints and whole-body deformations.

However, the change in pressure during the first measurement phase 10and during the second measurement phase 20 is typically symmetrical, asshown in the exemplary embodiment in FIG. 4. The symmetric configurationof the pressure profile during the first measurement phase and thesecond measurement phase is advantageous in that interfering effectswhich are based on vibrations are reduced in an even better way.

The phase images B₀ to B₂ are captured during the first measurementphase 10 in the exemplary embodiment shown in FIG. 4, the phase image B₃is captured at the transition between the first phase 10 and the secondphase 20 and the phase images B₄ to B₆ are captured during the secondmeasurement phase 20, with the phase image B₃ captured at the transitionbetween the first phase 10 and the second phase 20 again being used forboth phases. As already described with regard to the first exemplaryembodiment, partial phase difference images D_(i)=B_(i+1)−B_(i) are nowgenerated and summed from successive phase images, with the partialphase difference images from the second measurement phase 20 beingincluded in the summation with an opposite sign.

The use of the opposite sign for the second measurement phase 20 leadsto the partial phase difference images generated in said phase acting onthe representation in the overall phase difference image of thedeformations generated by faults in the tire in such a way as if thepressure were to continue to rise during the second measurement phaseand were to correspond to the line 25 plotted in a dashed manner in FIG.4 (this applies at least if the assumption of a linear relationship ismade between the pressure and the deformation of the tire). Since thisdoubles the overall effect, the magnitude of the change in pressure canbe reduced correspondingly, as shown in FIG. 4. By contrast, since theeffects caused by vibrations, pressure points and whole-bodydeformations are independent of pressure, the opposite sign for thefirst measurement phase and the second measurement phase leads to amutual reduction of these influences.

According to an exemplary embodiment of the disclosure, it is alsopossible to use a plurality of successive first measurement phases andsecond measurement phases, as shown in the third exemplary embodiment inFIG. 5.

Following a first measurement phase 10 with rising negative pressure anda second measurement phase 20 with falling negative pressure there is afurther first measurement phase 10′ with rising negative pressure andthen further second measurement phase 20′ with falling negativepressure. Further first measurement phases 10″ and further secondmeasurement phases 20″ may follow.

Typically, the same pressure profile is selected for all firstmeasurement phases 10, 10′, 10″ and the same pressure profile is alsoselected for all second measurement phases 20, 20′, 20″ such that,overall, this typically results in a periodic profile of the pressure.Typically, the pressure profile between the first measurement phases andthe second measurement phases is once again symmetrical.

The phase images and partial phase difference images are, in this casetoo, generated in the same manner as already described above for thefirst measurement phases and second measurement phases. Furthermore,here too, the partial phase difference images from the secondmeasurement phases 20, 20′ are included in the summation with theopposite sign to the partial phase difference images from the firstmeasurement phases 10, 10′, 10″.

In the exemplary embodiment shown in FIG. 5, a phase image is capturedand recorded, in each case, at a maximum and a minimum pressure, i.e.,at the start and end of the measurement and in the overlap regions ofthe first and second measurement phases, with the phase images B₂, B₄,B₆, and B₈ captured and recorded in the overlap regions of the first andsecond measurement phases being included in each case both in the lastphase difference image of the preceding measurement phase and in thefirst phase difference image of the subsequent measurement phase. Then,only one intermediate phase image B_(i), B₃, B₅, B₇, and B₉ still iscaptured and recorded within the individual measurement phases, and soonly two partial phase difference images are determined per measurementphase.

The effective overall pressure change included in an overall differenceimage is increased by the use of a plurality of first and secondmeasurement phases in the case of the same pressure change permeasurement phase. Accordingly, the change in pressure per measurementphase can be correspondingly reduced in the case of an unchanging oreven increasing quality of the tire test.

While changes in pressure in the region of 50 mbar are conventionalaccording to the prior art, the method according to the disclosureallows smaller changes in pressure. By way of example, work may becarried out with pressure changes that are less than 30 mbar, typicallyless than 20 and more typically less than 10 mbar. To this end, it isonly necessary to use correspondingly many first and second measurementphases and sum the corresponding partial phase difference images.

In the exemplary embodiments shown in FIGS. 3 to 5, a plurality ofpartial phase difference images are generated per measurement phase ineach case. However, the smaller the pressure changes per measurementphase are, the fewer partial phase difference images are required permeasurement phase. Accordingly, it is likewise conceivable to generateonly one partial phase difference image per measurement phase. In thiscase, it is only necessary to captured a phase image at, respectively,the start and end of each measurement phase or at, respectively, aminimum and a maximum pressure, and a partial phase difference imagemust be formed therefrom. Therefore, it is only necessary to capture aphase image at, respectively, the start and the end of the measurementand the transition between a first phase and a second phase or between asecond phase and a first phase.

The change in pressure is typically continuous during the individualmeasurement phases. In particular, use can be made here of asubstantially linear pressure profile. However, as illustrated in FIG.3, this is not mandatory.

In the exemplary embodiments shown in FIGS. 3 to 5, the first phaseimage of the first measurement phase and the last phase image of thesecond measurement phase are respectively captured and recorded atambient pressure. As a result, the entire time interval over which thepressure in the pressure chamber is changed can be used as part of ameasurement phase.

However, alternatively, the testing method may also operate with anoffset, as shown in FIGS. 6 and 7. To this end, the pressure in thepressure chamber is initially changed by an offset O from an initialpressure, which is usually the ambient pressure and hence atmosphericpressure, before the first measurement phase 10 sets in with a firstmeasurement image. In the same way, the second measurement phase may endwith the capturing and recording of the last phase image when the offsetO is reached.

The exemplary embodiment shown in FIG. 6 corresponds to the procedureknown from FIG. 4, with, however, the phase image B₁ being used as afirst image of the first measurement phase and the phase image B₅ beingused as the last phase image of the second measurement phase. As aresult, an offset O arises for both measurement phases.

By contrast, the exemplary embodiment shown in FIG. 7 corresponds to theprocedure known from FIG. 5, wherein, however, a pressure change 70 bythe offset O is already undertaken before the first measurement phase.Then, the pressure is changed between the offset O and the maximumpressure in the individual measurement phases.

Within the scope of the present disclosure, shearography is typicallyused as interferometric measurement method since it is particularlysuitable for the severe industrial conditions usually present whentesting tires.

Typically, the individual phase images are generated, according to anexemplary embodiment of the disclosure, from a single measurement image.This is advantageous in that there is no need to stop the change inpressure for producing a phase image. Instead, the phase images may alsobe generated during the change in pressure, optionally at a highfrequency.

Typically, the coherent radiation reflected by the tire is imaged ontoan image plane by an imaging optical unit for the purposes of capturingthe phase images, a sensor being located in said image plane, whereinreference radiation generated according to the shearing method issuperposed on the sensor and the phase of the radiation is determinedfrom the measurement signals of the sensor. To this end, the measuringhead typically operates on the basis of a spatial phase shift.

Typically, the imaging optical unit has a stop with at least oneaperture, in particular a slit. Typically, the stop has two apertures,in particular two slits. As a result, it is possible to generate aspatial carrier frequency such that it is possible to generate a phaseimage from only one measurement image. By way of example, the sensor isa charge-coupled device (CCD) sensor and/or a complementarymetal-oxide-semiconductor (CMOS) sensor. The imaging optical unit can beconfigured and the measurement can be effectuated as described in DE 19856 400 A1. The content of DE 198 56 400 A1 is incorporated, in itsentirety thereof, in the subject matter of the present application.

Furthermore, the individual partial phase difference images can besubjected to phase filtering before they are summed. A better coherencerelation and a better phase image quality emerge from phase-filteringthe individual partial phase difference images.

Furthermore, it is possible, according to an exemplary embodiment of thedisclosure, to eliminate or reduce the whole-body deformation or theinfluences thereof on the overall phase difference image. Furthermore,it is possible to eliminate or reduce the local deformation on accountof pressure points and the deformation on account of vibrations or theinfluences thereof on the overall phase difference image. In particular,this is effectuated by the summation over the second measurement phasewith an opposite sign.

Reducing the influences of a whole-body deformation can additionally beeffectuated by corresponding processing of the phase images, the partialphase difference images and/or the overall phase difference image. Byway of example, the overall phase difference image generated by thewhole-body deformation can be ascertained from reference measurements orfrom filtering the overall phase difference image and may be subtractedfrom the overall phase difference image.

Furthermore, the phase images and the partial phase difference imagescan be generated and/or the summation can be effected as described in DE101 01 057 A1. Therefore, the content of DE 10101057 A1, in itsentirety, is also incorporated in the subject matter of the presentapplication.

It is understood that the foregoing description is that of the exemplaryembodiments of the disclosure and that various changes and modificationsmay be made thereto without departing from the spirit and scope of thedisclosure as defined in the appended claims.

What is claimed is:
 1. A method for testing a tire by interferometry ina pressure chamber of a tire testing device, the method comprising:capturing phase images at different pressures in the pressure chamber;generating partial phase difference images between successive phaseimages; summing the partial phase difference images to form an overallphase difference image; changing a pressure in the pressure chamber in afirst direction during a first measurement phase; changing the pressurein a direction opposite to the first direction during a secondmeasurement phase; and including at least one partial phase differenceimage from the first measurement phase and at least one partial phasedifference image from the second measurement phase in the summing. 2.The method according to claim 1, further comprising: including a partialphase difference image or the partial phase difference images from thesecond measurement phase in the summing with an opposite sign.
 3. Themethod according to claim 1, further comprising: generating at least twopartial phase difference images; and including the at least two partialphase difference images in the summing during at least one of the firstmeasurement phase and the second measurement phase.
 4. The methodaccording to claim 1, wherein a time duration of the second measurementphase is at least 50% of the time duration of the first measurementphase, wherein a time interval between generating successive phaseimages over the first measurement phase and the second measurement phasevaries by at most 20% of the shortest time interval, and wherein thetime interval is invariable for all phase images.
 5. The methodaccording to claim 1, further comprising: changing the pressure duringthe second measurement phase more quickly than during the firstmeasurement phase; and venting or aerating the pressure chamber duringthe second measurement phase.
 6. The method according to claim 1,further comprising: symmetrically changing the pressure during thesecond measurement phase relative to the changing of the pressure duringthe first measurement phase.
 7. The method according to claim 1, furthercomprising: including a plurality of successive first and secondmeasurement phases, wherein the plurality of successive first and secondmeasurement phases forms a periodic pressure profile and/or absolutepressure changes of the plurality of successive first and secondmeasurement phases have a same magnitude.
 8. The method according toclaim 1, wherein the first measurement phase starts only after thepressure in the pressure chamber changes relative to an initialpressure, and/or wherein the second measurement phase ends before thepressure in the pressure chamber returns to the initial pressure.
 9. Themethod according to claim 1, wherein at least one phase image includesboth the partial phase difference image from the first measurement phaseand the partial phase difference image from the second measurementphase, and wherein a phase image is captured at a transition between twomeasurement phases, the phase image being included in a last partialphase difference image of a measurement phase lying before thetransition and in a first partial phase difference image of ameasurement phase lying after the transition.
 10. The method accordingto claim 1, further comprising: generating a negative pressure in thepressure chamber during the first measurement phase, the negativepressure being released during the second measurement phase.
 11. Themethod according to claim 1, further comprising: eliminating awhole-body deformation from the phase images and/or the partial phasedifference images and/or the overall phase difference image, and phasefiltering the partial phase difference images prior to the summing. 12.The method according to claim 1, further comprising: generating thephase images from a single measurement image; generating the partialphase shift difference images by a spatial phase shift; and generatingthe single measurement image by an imaging optical unit, the imagingoptical unit having a stop with at least one aperture configured togenerate a spatial carrier frequency.
 13. The method according to claim1, further comprising: continuously changing the pressure during a firstphase and/or a second phase; recording at least one of the phase imageswhile the pressure is changing; changing of the pressure during a tiretest less than 50 mbar.
 14. A tire testing device for testing a tire byinterferometry, the tire testing device comprising: a pressure chamber,at least one interferometric measuring head, and a controller configuredto: capture phase images at different pressures in the pressure chamber;generate partial phase difference images between successive phaseimages; sum the partial phase difference images to form an overall phasedifference image; change a pressure in the pressure chamber in a firstdirection during a first measurement phase; change the pressure in adirection opposite to the first direction during a second measurementphase; and include at least one partial phase difference image from thefirst measurement phase and at least one partial phase difference imagefrom the second measurement phase in the summing.
 15. A control softwarefor a tire testing device according to claim 14.